[llvm] [LoopVectorizer][ARM] Detect reduce(ext(mul(ext, ext))) patterns more reliably (PR #115847)

[Elvis Wang via llvm-commits]

ElvisWang123 wrote:

> @ElvisWang123 if this makes the cost model change easier or more difficult let me know. It should just bring the two cost-models closer together, but the new costmodel will get the benefit already. We can easily drop this if its simpler that way.

These test cases show how VPlan-based cost model can calculate cost more accurately. 
Without the assertion of checking VF between legacy and vplan-based, I think it is good to keep these tests.

https://github.com/llvm/llvm-project/pull/115847